Tanya Heath is on a double mission: to prove women can wear heels without ruining their feet, and that her answer to their plight, a heel that switches from high to low, can be made entirely in France.

The Paris-based Canadian started with a simple idea. When your shoes start to hurt -- half way through a party, a wedding or a workday -- press a button in the sole, slot out your dressy high heel and replace it with a walking version.

"I'm a feminine feminist," is how the 42-year-old sums up her philosophy. "My shoe is designed to be sexy -- but on the woman's terms."

"You can do your two-hour meeting, and then you just take off your high heel," she explained at the Ethical Fashion Show in Paris this month. "You're getting relief -- and you're getting home."

So far so good, except the trick -- which no one had quite figured out until now -- is how to keep the shoe balanced and comfortable both on tip-toe and when you tilt it back to sit on a low heel.

Fruit of three painstaking years of research, Heath's patented answer to the riddle is billed as the world's first multi-height heel, a luxury leather shoe that switches seamlessly from nine to four centimetres (3.5 to 1.5 inches).

From pastel pink patent sandals to strappy dancing shoes or demure lace-ups, with either stiletto or chunky heels, high or low, the shoes are pitched at the high end of the market, starting at around 250 euros (320 dollars).

With models harking back to the 1920s, Heath wanted a "deliberately nostalgic" style to offset the "gee-whizz technology".

"It's an incredible game of geometry," she said. "All shoes, quality ones, follow a set of geometric rules, and always have done. I don't follow those rules. We did things differently."

A passionate heel-wearer, Heath's project was born partly of personal experience, having suffered foot deformations from heels, like an estimated 38% of women worldwide.

"I had had enough of aching feet, and I refused point blank to wear ballerina flats," she joked.

But she she also wanted to show shoes could still be made in a high-cost economy like France.

'I HAD HAD ENOUGH OF ACHING FEET'

In 1996, Heath left a job as policy analyst at the Canadian foreign ministry for a new life in Paris, following her oil executive fiance, a "camembert and champagne" lover who refused to be based anywhere else.

Once there she learnt French at business school, worked in management and high-tech, then private equity, all while raising three young children.

In 2009 she quit her job and threw herself into the heel project, heading to the Dordogne to investigate taking over a struggling shoe factory as a way to kickstart the project.

When she got there Heath was in for a shock.

"I saw the factory closed down and 52 people out on the street, the boss locked out -- and I thought 'Woah!'

As a liberally trained economist, she had two ways of reading the situation.

"One is that France is non-competitive, and we deserve everything we get because we've killed the industry. And the other way is to say maybe, with an innovation, we can save some jobs here.

The buyout idea came to nothing, but she went on to surround herself with a team of 21 engineers, designers and technicians, plus herself and an associate, to bring her concept to life.

The resulting shoe is entirely made in France, with leather from central Limoges, produced in three factories, in eastern Franche-Compte for the mechanism, western Cholet for the heel and northwestern Rouen for the body.

Since they started shipping in February, Heath has sold 800 pairs via a retail corner in the Printemps department store, her website, the Sarenza online store, and half a dozen boutiques across five countries.

She says running a factory in Rouen -- as opposed to China or Vietnam -- gives her an edge in fashion terms.

"I can prototype much more quickly, do capsule collections. I made my colour decisions for this winter just two weeks ago. And I can tell you now that the colours for this winter are midnight blue, black, wine red and jaguar green!"

With 30 shoe references and 120 heels, Heath is one of around 50 firms including Repetto and Arche, who still make shoes entirely in France.

But while her upscale 'Tanya Heath' line will continue to be made in France, she already has plans for a second brand, which "will have to be made somewhere else".

To meet large orders coming in from South Korea or Canada, she needs to scale up production and bring down her price.

"I would like a real working girl to be able to have them," Heath said. "But it's impossible to make a Made in France affordable. A skin of leather alone costs 80 euros here. I'm here to tell you it can't be done."